Devices for measurement of microwave power are either thermoelectric or rectifying. Thermoelectric devices operate by allowing the input signal power to heat an electric element that changes resistance with temperature. A delicately balanced electronic bridge circuit detects this change which is applied to a meter after some amplification. The thermoelectric devices are generally complex and require careful calibration in order to detect r.f. power levels as low as -20 DBM. Rectifying devices employ detector diodes which by virtue of their square law response provide a d.c. output with about 0.4 MV/W. The output signal must, therefore, be amplified before it can activate any visual or audio indicators. CW or FM signals are difficult to detect by diode detectors due to amplifier stability requirements. This can be circumvented by pre-modulating the input signal before detection. This technique, however, is also relatively complex and costly. Currently available field intensity meters are likewise very costly.